Polishing pad

ABSTRACT

The invention provides a polishing pad comprising a polishing layer having a polishing surface comprising plurality of grooves disposed into the polishing layer a measurable depth from the polishing surface, and a barrier region free of grooves, and a transparent window disposed in and surrounded by the barrier region.

BACKGROUND OF THE INVENTION

Chemical-mechanical polishing (“CMP”) processes are used in themanufacturing of microelectronic devices to form flat surfaces onsemiconductor wafers, field emission displays, and many othermicroelectronic substrates. For example, the manufacture ofsemiconductor devices generally involves the formation of variousprocess layers, selective removal or patterning of portions of thoselayers, and deposition of yet additional process layers above thesurface of a semiconducting substrate to form a semiconductor wafer. Theprocess layers can include, by way of example, insulation layers, gateoxide layers, conductive layers, layers of metal or glass, etc. It isgenerally desirable in certain steps of the wafer process that theuppermost surface of the process layers be planar, i.e., flat, for thedeposition of subsequent layers. CMP is used to planarize process layerswherein a deposited material, such as a conductive or insulatingmaterial, is polished to planarize the wafer for subsequent processsteps.

In a typical CMP process, a wafer is mounted upside down on a carrier ina CMP tool. A force pushes the carrier and the wafer downward toward apolishing pad. The carrier and the wafer are rotated above the rotatingpolishing pad on the CMP tool's polishing table. A polishing composition(also referred to as a polishing slurry) generally is introduced betweenthe rotating wafer and the rotating polishing pad during the polishingprocess. The polishing composition typically contains a chemical thatinteracts with or dissolves portions of the uppermost wafer layer(s) andan abrasive material that physically removes portions of the layer(s).The wafer and the polishing pad can be rotated in the same direction orin opposite directions, whichever is desirable for the particularpolishing process being carried out. The carrier also can oscillateacross the polishing pad on the polishing table.

In polishing the surface of a wafer, it is often advantageous to monitorthe polishing process in situ. One method of monitoring the polishingprocess in situ involves the use of a polishing pad having an apertureor window. The aperture or window provides a portal through which lightcan pass to allow the inspection of the wafer surface during thepolishing process. Polishing pads having apertures and windows are knownand have been used to polish substrates, such as the surface ofsemiconductor devices. For example, U.S. Pat. No. 5,605,760 provides apad having a transparent window formed from a solid, uniform polymer,which has no intrinsic ability to absorb or transport slurry. U.S. Pat.No. 5,433,651 discloses a polishing pad wherein a portion of the pad hasbeen removed to provide an aperture through which light can pass. U.S.Pat. Nos. 5,893,796 and 5,964,643 disclose removing a portion of apolishing pad to provide an aperture and placing a transparentpolyurethane or quartz plug in the aperture to provide a transparentwindow, or removing a portion of the backing of a polishing pad toprovide a translucency in the pad. U.S. Pat. Nos. 6,171,181 and6,387,312 disclose a polishing pad having a transparent region that isformed by solidifying a flowable material (e.g., polyurethane) at arapid rate of cooling.

A problem often encountered during chemical-mechanical polishing is thetendency for the polishing composition and the resulting polishingdebris to accumulate at the polishing pad window. The accumulatedpolishing composition and polishing debris can obstruct transmission oflight through the window thereby reducing the sensitivity of the opticalendpoint detection method.

Although several of the above-described polishing pads are suitable fortheir intended purpose, a need remains for other polishing pads thatprovide effective planarization coupled with effective optical endpointdetection, particularly in the chemical-mechanical polishing of asubstrate. In addition, there is a need for polishing pads havingsatisfactory features such as polishing efficiency, slurry flow acrossand within the polishing pad, resistance to corrosive etchants, and/orpolishing uniformity. Finally, there is a need for polishing pads thatcan be produced using relatively low cost methods and which requirelittle or no conditioning prior to use.

BRIEF SUMMARY OF THE INVENTION

The invention provides a polishing pad comprising (a) a polishing layerhaving a polishing surface comprising (1) a plurality of groovesdisposed into the polishing layer a measurable depth from the polishingsurface, and (2) a barrier region free of grooves, and (b) a transparentwindow disposed in and surrounded by the barrier region. The inventionfurther provides a method of polishing a substrate comprising (i)providing a workpiece to be polished, (ii) contacting the workpiece withthe polishing pad of the invention, and (iii) abrading at least aportion of the surface of the workpiece with the polishing system topolish the workpiece.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic top view illustrating a polishing pad of theinvention having a polishing layer (10) comprising a polishing surface(12) with continuous grooves (40) and discontinuous grooves (50) and asubstantially transparent window (15) disposed in a barrier region (20)substantially free of grooves.

FIG. 2 is a fragmentary, partially cross-sectional perspective viewdepicting a polishing pad of the invention having a polishing layer (10)comprising a polishing surface (12) with continuous grooves (40) anddiscontinuous grooves (50) disposed into the polishing layer (10) ameasurable depth from the polishing surface and a substantiallytransparent window (15) disposed in a barrier region (20) substantiallyfree of grooves.

FIG. 3 is a fragmentary, partially cross-sectional perspective viewdepicting a polishing pad of the invention having a polishing layer (10)comprising a polishing surface (12) with continuous grooves (40) anddiscontinuous grooves (50) disposed into the polishing layer ameasurable depth from the polishing surface and a substantiallytransparent window (15) disposed in a barrier region (20) free ofgrooves, wherein the depth of the discontinuous grooves transition froma maximum value to zero proximate to the barrier region (20).

FIG. 4 is a fragmentary, partially cross-sectional perspective viewdepicting a polishing pad of the invention having a polishing layer (10)and a sub-layer (30), wherein the sub-layer has an aperture (35)substantially aligned with the transparent window (15).

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a chemical-mechanical polishing padcomprising a polishing layer and a transparent window. The polishinglayer has a polishing surface comprising (a) a plurality of groovesdisposed into the polishing layer and (b) a barrier region free ofgrooves. The transparent window is disposed in and surrounded by thebarrier region. While not wishing to be bound to any particular theory,it is believed that the presence of a barrier region substantially freeof grooves surrounding the transparent window will reduce the amount ofpolishing composition that remains on or within the transparent window.

The polishing pad can have any suitable dimensions. Typically, thepolishing pad will be circular in shape (as is used in rotary polishingtools) or will be produced as a looped linear belt (as is used in linearpolishing tools). Preferably, the polishing pad is circular.

FIG. 1 depicts a circular polishing pad of the invention comprising apolishing layer (10) comprising a polishing surface (12), continuousgrooves (40) and discontinuous grooves (50), and a transparent window(15) disposed in a barrier region (20) substantially free of grooves.

The grooves disposed in the polishing layer facilitate the lateraltransport of polishing compositions across the surface of the polishingpad. The grooves can be in any suitable pattern. For example, groovescan be in the form of slanted grooves, circular grooves, concentricgrooves, spiral grooves, radial grooves, or XY crosshatch pattern. Thepolishing pad can have two or more different groove patterns. Forexample, the polishing pad can have a combination of concentric groovesand radial grooves or a combination of concentric grooves and XYcrosshatch grooves. Preferably, the grooves are concentric.

The grooves can be continuous in connectivity across the surface of thepolishing pad. Alternatively, the grooves can be discontinuous acrossthe surface of the pad wherein each discontinuous groove has a first endand a second end. In one embodiment, as depicted in FIG. 1, a portion ofthe grooves, i.e., one or more grooves, are continuous (40), and aportion of the grooves, i.e., one or more grooves, are discontinuous(50).

As depicted in FIG. 2, each groove has a measurable depth from thepolishing surface. The depth can be any suitable depth. For example, thedepth can be 1 mm or less, 0.8 mm or less, 0.6 mm or less, 0.5 mm orless, 0.4 mm or less, 0.3 mm or less, 0.2 mm or less, or 0.1 mm or less.The depth of each continuous and/or discontinuous groove can be constantor can vary along the length or the circumference of the grooves. In oneembodiment, as shown in FIG. 3, the depth of a discontinuous groove (50)transitions from a maximum depth to zero, i.e., the groove tapers to thepolishing surface (12) at least one end of the discontinuous groove.Preferably, the depth transition occurs gradually, i.e., over a lengthof about 0.5 mm or more, 1 mm or more, about 2 mm or more, about 3 mm ormore, about 4 mm or more, or about 5 mm or more. In one embodiment, thedepth transition from a maximum depth to zero occurs proximate to thebarrier region.

The polishing pad further comprises a barrier region which issubstantially free of grooves or desirably merely free of grooves. Inone embodiment, as shown in FIG. 1, the barrier region (20) is disposedbetween the first and second ends of concentric, discontinuous grooves(50).

The barrier region can have any suitable dimensions and any suitableshape. The perimeter of the barrier region, defined by adjacentcontinuous and/or discontinuous grooves, can be any suitable shape(e.g., round, oval, square, rectangular, triangular, and so on). Whenthe barrier region is oval or rectangular in shape, the barrier regiontypically has a long diameter axis or length of about 2 cm or more(e.g., about 3 cm or more, about 4 cm or more about 5 cm or more, about6 cm or more, about 7 cm or more, about 8 cm or more, about 9 cm ormore, about 10 cm or more, about 11 cm or more, or about 12 cm or more)and a short diameter axis or width of about 1 cm or more (e.g., about 2cm or more, about 3 cm or more, about 4 cm or more, about 5 cm or more,about 6 cm or more, about 7 cm or more, or about 8 cm or more). When thebarrier region is circular or square in shape, the perimeter of thebarrier region typically has a diameter or width of about 2 cm or more(e.g., about 3 cm or more, about 4 cm or more, about 5 cm or more, about6 cm or more, about 7 cm or more, about 8 cm or more, about 9 cm ormore, or about 10 cm or more)]

As depicted in FIG. 1, the transparent window (15) is disposed in andsurrounded by the barrier region (20). The transparent window can bedisposed symmetrically or asymmetrically in the barrier region.

The transparent window will have a perimeter that defines thetransparent window in the barrier region. Each point on the perimeter ofthe transparent window will have a shortest distance L to a continuousand/or discontinuous groove, thereby providing a series of shortestdistances or spacings from the perimeter of the transparent window tothe grooves (i.e., L₁, L₂, L₃, . . . ). The smallest value (L_(min)) andthe largest value (L_(max)) of the series of shortest distances can beany suitable length. For example, L_(min) can be about 0.5 cm or more(e.g., about 1 cm or more, about 1.5 cm or more, about 2 cm or more,about 3 cm or more, or about 4 cm or more). In addition, oralternatively, L_(min) can be 10 cm or less (e.g., about 8 cm or less,about 7 cm or less, about 6 cm or less, about 5 cm or less, or about 4cm or less). L_(max) can be independently about 0.5 cm or more (e.g.,about 1 cm or more, about 1.5 cm or more, about 2 cm or more, about 3 cmor more, or about 4 cm or more). In addition, or alternatively, theL_(max) can be independently 10 cm or less (e.g., about 8 cm or less,about 7 cm or less, about 6 cm or less, about 5 cm or less, or about 4cm or less). L_(min) and L_(max) can be the same or different withrespect to, independently, continuous grooves and discontinuous grooves.Moreover, the series of shortest distances L₁, L₂, L₃, etc. need not bethe same such that there is uniform spacing between the transparentwindow and the grooves.

The transparent window can have any suitable dimensions (e.g., length,width, diameter, and thickness) and any suitable shape (e.g., round,oval, square, rectangular, triangular, and so on). When the transparentwindow is oval or rectangular in shape, the transparent window typicallyhas a long diameter axis or length of about 1 cm or more (e.g., about 2cm or more, about 3 cm or more, about 4 cm or more, about 5 cm, about 6cm or more, about 7 cm or more, or about 8 cm or more) and a shortdiameter axis or width of about 0.5 or more (e.g., about 1 cm or more,about 1.5 cm or more, about 2 cm or more, about 3 cm or more, or about 4cm or more). When the transparent window is circular or square in shape,the transparent window typically has a diameter or width of about 1 cmor more, (e.g., about 2 cm or more, about 3 cm or more, about 4 cm ormore, or about 5 cm or more).

The transparent window (15) comprises an optically transmissivematerial. The presence of the transparent window enables the polishingpad to be used in conjunction with an in situ CMP process monitoringtechnique. Typically, the optically transmissive material has a lighttransmission of at least about 10% or more (e.g., about 20% or more,about 30% or more, or about 40% or more) at one or more wavelengths offrom about 190 nm to about 10,000 nm (e.g.; about 190 nm to about 3500nm, about 200 nm to about 1000 nm, or about 200 nm to about 780 nm). Theoptically transmissive material can be any suitable material, many ofwhich are known in the art. The optically transmissive material can bethe same or different than the material in the remainder of thepolishing pad. For example, the optically transmissive material canconsist of a glass or polymer-based plug that is inserted in an apertureof the polishing pad or can comprise the same polymeric material used inthe remainder of the polishing pad.

The transparent window can be affixed to the polishing pad by anysuitable means. For example, the transparent window can be affixed tothe polishing pad through the use of an adhesive. Desirably, thetransparent window is affixed to the polishing layer without the use ofan adhesive, for example by welding. Similarly, the transparent windowcan have any suitable structure (e.g., crystallinity), density, andporosity. For example, the transparent window can be solid or porous(e.g., microporous or nanoporous having an average pore size of lessthan 1 micron). Preferably, the transparent window is solid or is nearlysolid (e.g., has a void volume of about 3% or less).

As depicted in FIG. 2, the transparent window (15) can be flush with thepolishing surface (12) of the polishing layer, i.e., the upper mostwindow surface is substantially co-planar with the polishing surface.Alternatively, the transparent window can be recessed from the polishingsurface of the polishing layer.

The polishing layer can be used alone or optionally as one layer of amulti-layer stacked polishing pad. For example, as shown in FIG. 4, thepolishing layer (10) comprising a polishing surface (12), a barrierregion substantially free of grooves (20), continuous grooves (40) anddiscontinuous grooves (50), and a transparent window (15) can be used incombination with a sub-layer (30) that is substantially coextensive withthe polishing layer. In some embodiments, the sub-layer (30) comprisesan aperture (35) that is substantially aligned with the transparentwindow (15) of the polishing layer.

The polishing layer, the barrier region, the transparent window, and thesub-layer of the polishing pad can comprise any suitable material, whichcan be the same or different. Desirably, the polishing layer, thebarrier region, the transparent window, and the sub-layer of thepolishing pad each independently comprise a polymer resin. The polymerresin can be any suitable polymer resin. Typically, the polymer resin isselected from the group consisting of thermoplastic elastomers,thermoset polymers, polyurethanes (e.g., thermoplastic polyurethanes),polyolefins (e.g., thermoplastic polyolefins), polycarbonates,polyvinylalcohols, nylons, elastomeric rubbers, elastomericpolyethylenes, polytetrafluoroethylenes, polyethyleneterephthalates,polyimides, polyaramides, polyarylenes, polyacrylates, polystyrenes,polymethylmethacrylates, copolymers thereof, and mixtures thereof.Preferably, the polymer resin is polyurethane, more preferablythermoplastic polyurethane. The polishing layer, the barrier region, thetransparent window, and the sub-layer can comprise a different polymerresin. For example, the polishing layer can comprise porous thermosetpolyurethane, the sub-layer can comprise closed-cell porous polyurethanefoam, and the transparent window can comprise solid thermoplasticpolyurethane.

The polishing layer and the sub-layer typically will have differentchemical (e.g., polymer composition) and/or physical properties (e.g.,porosity, compressibility, transparency, and hardness). For example, thepolishing layer and the sub-layer independently can be closed cell(e.g., a porous foam), open cell (e.g., a sintered material), or solid(e.g., cut from a solid polymer sheet). Preferably the polishing layeris less compressible than the sub-layer. The polishing layer and thesub-layer can be formed by any suitable method, many of which are knownin the art. Suitable methods include casting, cutting, reactioninjection molding, injection blow molding, compression molding,sintering, thermoforming, and pressing the porous polymer into thedesired polishing pad shape. Other polishing pad elements also can beadded to the porous polymer before, during, or after shaping the porouspolymer, as desired. For example, backing materials can be applied,holes can be drilled, or surface textures can be provided (e.g.,grooves, channels), by various methods generally known in the art.

The polishing layer optionally further comprise organic or inorganicparticles. For example, the organic or inorganic particles can beselected from the group consisting of metal oxide particles (e.g.,silica particles, alumina particles, ceria particles), diamondparticles, glass fibers, carbon fibers, glass beads, aluminosilicates,phyllosilicates (e.g., mica particles), cross-linked polymer particles(e.g., polystyrene particles), water-soluble particles, water-absorbentparticles, hollow particles, combinations thereof, and the like. Theparticles can have any suitable size. For example, the particles canhave an average particle diameter of about 1 nm to about 10 microns(e.g., about 20 nm to about 5 microns). The amount of the particles inthe body of the polishing pad can be any suitable amount, for example,from about 1 wt. % to about 95 wt. % based on the total weight of thepolishing pad body.

The polishing pad of the invention is particularly suited for use inconjunction with a chemical-mechanical polishing (CMP) apparatus.Typically, the apparatus comprises a platen, which, when in use, is inmotion and has a velocity that results from orbital, linear, or circularmotion, a polishing pad of the invention in contact with the platen andmoving with the platen when in motion, and a carrier that holds aworkpiece to be polished by contacting and moving relative to thesurface of the polishing pad. The polishing of the workpiece takes placeby the workpiece being placed in contact with the polishing pad and thenthe polishing pad moving relative to the workpiece, typically with apolishing composition therebetween, so as to abrade at least a portionof the workpiece to polish the workpiece. The polishing composition canbe any suitable polishing composition. A polishing composition typicallycomprises a liquid carrier (e.g., an aqueous carrier), a pH adjustor,and an abrasive. Depending on the type of workpiece being polished, thepolishing composition optionally further comprises oxidizing agents,organic acids, complexing agents, pH buffers, surfactants, corrosioninhibitors, anti-foaming agents, and the like. The CMP apparatus can beany suitable CMP apparatus, many of which are known in the art. Thepolishing pad of the invention also can be used with linear polishingtools.

Desirably, the CMP apparatus further comprises an in situ polishingendpoint detection system, many of which are known in the art.Techniques for inspecting and monitoring the polishing process byanalyzing light or other radiation reflected from a surface of theworkpiece are known in the art. Such methods are described, for example,in U.S. Pat. No. 5,196,353, U.S. Pat. No. 5,433,651, U.S. Pat. No.5,609,511, U.S. Pat. No. 5,643,046, U.S. Pat. No. 5,658,183, U.S. Pat.No. 5,730,642, U.S. Pat. No. 5,838,447, U.S. Pat. No. 5,872,633, U.S.Pat. No. 5,893,796, U.S. Pat. No. 5,949,927, and U.S. Pat. No.5,964,643. Desirably, the inspection or monitoring of the progress ofthe polishing process with respect to a workpiece being polished enablesthe determination of the polishing end-point, i.e., the determination ofwhen to terminate the polishing process with respect to a particularworkpiece.

The polishing pad of the invention is suitable for use in a method ofpolishing many types of workpieces (e.g., substrates or wafers) andworkpiece materials. For example, the polishing pad can be used topolish workpieces including memory storage devices, glass substrates,memory or rigid disks, metals (e.g., noble metals), magnetic heads,inter-layer dielectric (ILD) layers, polymeric films, low and highdielectric constant films, ferroelectrics, micro-electro-mechanicalsystems (MEMS), semiconductor wafers, field emission displays, and othermicroelectronic substrates, especially microelectronic substratescomprising insulating layers (e.g., metal oxide, silicon nitride, or lowdielectric materials) and/or metal-containing layers (e.g., copper,tantalum, tungsten, aluminum, nickel, titanium, platinum, ruthenium,rhodium, iridium, alloys thereof, and mixtures thereof). The term“memory or rigid disk” refers to any magnetic disk, hard disk, rigiddisk, or memory disk for retaining information in electromagnetic form.Memory or rigid disks typically have a surface that comprisesnickel-phosphorus, but the surface can comprise any other suitablematerial. Suitable metal oxide insulating layers include, for example,alumina, silica, titania, ceria, zirconia, germania, magnesia, andcombinations thereof. In addition, the workpiece can comprise, consistessentially of, or consist of any suitable metal composite. Suitablemetal composites include, for example, metal nitrides (e.g., tantalumnitride, titanium nitride, and tungsten nitride), metal carbides (e.g.,silicon carbide and tungsten carbide), nickel-phosphorus,alumino-borosilicate, borosilicate glass, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), silicon/germanium alloys, andsilicon/germanium/carbon alloys. The workpiece also can comprise,consist essentially of, or consist of any suitable semiconductor basematerial. Suitable semiconductor base materials include single-crystalsilicon, poly-crystalline silicon, amorphous silicon,silicon-on-insulator, and gallium arsenide.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A polishing pad comprising: (a) a polishing layer having a polishingsurface, wherein the polishing surface comprises: (1) a plurality ofgrooves disposed into the polishing layer a measurable depth from thepolishing surface, and (2) a barrier region free of grooves, and (b) atransparent window disposed in and surrounded by the barrier region. 2.The polishing pad of claim 1, wherein the depth transitions from amaximum value to zero proximate to the barrier region.
 3. The polishingpad of claim 2, wherein the depth transition is gradual.
 4. Thepolishing pad of claim 3, wherein the polishing pad is circular and thegrooves are concentric.
 5. The polishing pad of claim 4, wherein one ormore of the concentric grooves are discontinuous and have a first endand a second end, and wherein the barrier region is disposed between thefirst and second ends.
 6. The polishing pad of claim 1, wherein thetransparent window has an upper most window surface substantiallyco-planar with the polishing surface.
 7. The polishing pad of claim 1,wherein the polishing layer comprises polyurethane.
 8. The polishing padof claim 1, wherein the transparent window comprises thermoplasticpolyurethane.
 9. The polishing pad of claim 1, wherein the polishing padfurther comprises a sub-layer.
 10. A method of polishing a substratecomprising (i) providing a workpiece to be polished, (ii) contacting theworkpiece with the polishing pad of claim 1, and (iii) abrading at leasta portion of the surface of the workpiece with the polishing pad topolish the workpiece.
 11. The method of claim 10, wherein the methodfurther comprises detecting in situ a polishing endpoint.
 12. The methodof claim 10, wherein the workpiece is contacted with achemical-mechanical polishing composition located between the workpieceand the polishing pad.
 13. The method of claim 10, wherein the depthtransitions from a maximum value to zero proximate to the barrierregion.
 14. The method of claim 13, wherein the depth transition isgradual.
 15. The method of claim 14, wherein the polishing pad iscircular and the grooves are concentric.
 16. The method of claim 15,wherein one or more of the concentric grooves are discontinuous and havea first end and a second end, and wherein the barrier region is disposedbetween the first and second ends.
 17. The method of claim 10, whereinthe transparent window has an upper most window surface substantiallyco-planar with the polishing surface.
 18. The method of claim 10,wherein the polishing layer comprises polyurethane.
 19. The method ofclaim 10, wherein the transparent window comprises thermoplasticpolyurethane.
 20. The method of claim 10, wherein the polishing padfurther comprises a sub-layer.